The invention relates to NOx adsorber catalyst systems for reduction of the amount of hydrocarbons, carbon monoxide, and nitrogen oxides in automotive exhaust gases from lean burn and direct injection gasoline engines, and, more particularly, a trimetallic catalyst system for improved performance.
In order to meet government mandated exhaust gas emission standards, the exhaust gases of an automotive internal combustion engine must be treated before emission into the atmosphere. Typically, exhaust gases are routed through a catalytic converter device disposed in the exhaust outlet system of the engine, where the gases are treated by reactions with a catalyst deposited on a porous support material. The exhaust gases generally contain undesirable emission components including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxide (NOx). As a means of simultaneously removing the objectionable CO, HC, and NOx components, various xe2x80x9cthree-wayxe2x80x9d catalysts have been developed. Such catalysts typically employ one or more noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh), disposed on an alumina support. Other metals and metal oxides may also be included. Additional promoters such as rare earths, in particular cerium (Ce) and lanthanum (La), as well as alkaline-earth metals, such as barium (Ba) have been suggested to improve catalyst performance.
The need for fuel economy and the reduction of carbon dioxide (CO2) emissions now have made it generally desirable to operate internal combustion engines under xe2x80x9clean-burnxe2x80x9d and direct injection conditions. Under lean-burn conditions, the air-to-fuel ratio is adjusted to be somewhat greater than the stoichiometric ratio (about 14.7), generally between about 19 and about 35, in order to realize a fuel economy benefit. However, when operating under lean-burn conditions, typical three-way catalyst systems are efficient in oxidizing unburned HC and CO, but are inefficient in reducing NOx emission components.
One approach for treating nitrogen oxides in exhaust gases of engines operating under lean-burn conditions has been to incorporate NOx adsorbers in exhaust lines along with three-way catalysts. Such NOx adsorbers need to be able to operate within a wide temperature window of NOx control, while functioning as an effective three-way catalyst featuring good light-off as well as HC, CO, and NOx conversion performances. These adsorbers generally comprise a catalytic metal, which typically is platinum alone or which often further includes palladium and rhodium, in combination with an alkali or alkaline earth element or combinations thereof. The catalytic material in the adsorber acts first to oxidize NO to NO2; NO2 then reacts with the alkali and alkaline earth materials to form stable nitrate salts. In a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released for catalysis, whereupon NOx is reduced to N2 gas.
The catalyst materials are typically loaded onto a washcoat, e.g. a porous support material suitable for use in high temperature environments. As is common in the art, suitable washcoat materials are high surface area materials like alumina, gamma-alumina, zirconia, cerium oxide (ceria), and magnesium oxide, among others. For practical incorporation of the washcoat materials and the catalytic materials into internal combustion engine exhaust systems, the washcoat will, itself, be deposited on a chemically stable substrate material. Particularly useful substrate materials, as are commonly used for washcoat deposition, include cordierite and mullite, among others. The substrate may be of any size or shape, such as is required by the physical dimensions of the designed exhaust system. Similarly, the internal configuration of the substrate may be any known or commonly employed configuration. Substrates are typically formed as monolithic honeycomb structures, layered materials, or spun fibers, among other configurations.
However, there continues to be a need to develop a NOx adsorber catalyst system to provide effective exhaust gas treatment over the range of internal combustion engine operating conditions. Catalyst formulations including a Ba component, along with noble metals such as Pt and Rh, are reported to display favorable fresh and aged NOx adsorption performance at exhaust gas temperatures less than about 400xc2x0 C. However, these formulations disadvantageously appear to exhibit poor NOx treatment performance at higher operating temperatures (e.g., greater than about whether the catalyst formulation is fresh or aged.
It further is known that KNO3 thermodynamically is more stable than Ba(NO3)2. Accordingly, it has been proposed to incorporate K into a catalyst formulation, and, as expected, the addition of K serves significantly to improve high temperature NOx adsorption performance. Unfortunately, K also is known to deactivate noble metal catalyst components, and, as a result thereof, the incorporation of K into a NOx adsorber catalyst formulation negatively affects the noble metal activity. For example, light off characteristics of typical Pt and Rh components are detrimentally affected; and, in general, HC reduction performance of the catalyst formulations is adversely impacted.
Now, an improved NOx adsorber catalyst system has been developed for treatment of gases in an exhaust discharge of an internal combustion engine. It unexpectedly has been found that a catalyst system comprising a novel dual NOx adsorber catalyst system affords the advantages of the individual catalyst formulations of each NOx adsorber formulation, while minimizing the disadvantages of each individual formulation. This dual NOx adsorber catalyst system serves to optimize the NOx operating temperature window, HC performance, and catalyst light-off performance, as well as system sulfur release performance.
According to the present invention, a dual NOx adsorber catalyst system is employed, wherein a first adsorber catalyst formulation comprises effective amounts of a Ba component along with at least one noble metal component. The noble metal component(s) preferably is selected from the group consisting of Pt, Pd, and Rh. A second adsorber catalyst formulation is employed comprising effective amounts of both a Ba and a high temperature NOx adsorbent component along with at least one noble metal component. The high temperature NOx adsorbent component preferably is selected from the group consisting of Na, Cs, and K components; a K component is particularly preferred. Preferably, the noble metal component(s) is selected from the group consisting of Pt, Pd, and Rh. The amount of Ba component in the first and second catalyst formulations generally ranges from about 180 to about 2904 g/ft3, preferably about 363 to about 1452 g/ft3. The amount of high temperature NOx adsorbent component in the second catalyst formulation generally ranges from about 70 to about 1104 g/ft3, preferably about 138 to about 552 g/ft3. In regard to the noble metal components, in the first catalyst formulation Pt generally ranges from about 30 to 150 g/ft3, preferably about 50 to 110 g/ft3; Pd generally ranges from about 10 to about 80 g/ft3, preferably about 20 to about 80 g/ft3; Rh generally ranges from about 3 to about 30 g/ft3, preferably about 3 to about 15 g/ft3. In the second catalyst formulation, Pt generally ranges from about 20 to about 150 g/ft3, preferably about 50 to about 110 g/ft3; Pd generally ranges from about 10 to about 80 g/ft3; preferably about 10 to about 50 g/ft3; Rh generally ranges from about 3 to about 30 g/ft3, preferably about 5 to about 15 g/ft3.
The combination of a catalyst formulation with Ba, featuring good catalyst light-off characteristics and favorable low temperature NOx adsorption performance, with a Ba and K containing catalyst formulation, featuring good high temperature NOx adsorber performance, accomplishes a catalyst system with overall performance which cannot be achieved by either of the technologies individually.
Preferably, the first catalyst formulation comprises a Ba component and at least two noble metal components. More preferably, the first catalyst formulation is a trimetallic catalyst formulation. In particular, it is preferred that the first catalyst formulation comprise a Ba component, along with Pt, Rh, and Pd components. The Ba component preferably is in the form of BaCO3, BaO, or other active forms of Ba.
It is believed that the incorporation of a Pd component into the Ba catalyst formulation further serves to improve fresh, and more importantly, aged low temperature NOx adsorber performance, as well as stoichiometric light-off. Accordingly, a preferred first catalyst formulation comprises Ba, Pt, Rh, and Pd.
Preferably, the second catalyst formulation comprises a K component, a Ba component, and at least two noble metal catalyst components. More preferably, the second catalyst formulation is a trimetallic catalyst formulation. In particular, it is preferred that the second catalyst formulation comprise a Ba component and a K component along with a Pt component, and an Rh component, and a Pd component. The Ba component preferably is in the form of BaCO3, BaO, or other active forms of Ba; the K component preferably is in the form of K2CO3, K2O, or other active forms of K. It is believed that the further incorporation of Pd into this Ba and K containing trimetallic catalyst formulation serves to improve low temperature NOx performance, as well as improve the HC removal performance of the catalyst.
As discussed above, the incorporation of K into a NOx adsorber catalyst formulation is understood to result in poisoning deactivation of the noble metal catalyst components. Accordingly, the separation of a two adsorber catalyst system pursuant to the present invention serves to avoid poisoning impact on the first adsorber catalyst which does not contain K. To avert possible migration of K from the second K-containing catalyst adsorber to the first adsorber catalyst, it is preferred that the second adsorber catalyst be located in the exhaust gas outlet system at a point downstream of the first adsorber catalyst which contains Ba but no K. The upstream Ba-containing catalyst also serves the beneficial purpose of acting as a sulfur trap for the second adsorber catalyst, since BaSO4 is thermodynamically less stable than K2SO4, and trapped sulfur can be released under less severe conditions.